The ability to precisely control the molecular weight and molecular weight distributions, as well as gain sequence and architecture control in polymer synthesis is of considerable importance in a variety of technologies. For example, the development of controlled polymerization methods has profoundly changed polymer research with strategies, such as nitroxide-mediated radical polymerization (NMP), atom transfer radical polymerization (ATRP), and reversible addition fragmentation chain transfer polymerization (RAFT), processes that allow the facile synthesis of well-defined polymers that are diverse in both their structure and function (see, e.g. C. J. Hawker, et al. Chem. Rev. 2001, 101, 3661-3688).
Understandably, there have been a number of efforts to increase the technical applicability of polymerization processes, for example through strategies to regulate the activation and deactivation steps by using an external stimulus. However, there are few descriptions in the art of this type of regulation being successfully achieved through redox-controlled processes, electrochemical techniques, mechanochemical methods, or the allosteric control of catalysis. One strategy that controls both the initiation and growth exploits the unique aspects of electrochemistry to control the ratio of activator to deactivator in ATRP (see, e.g. A. J. D. Magenau, et al. Science 2011, 332, 81-84). With the selective targeting of redox-active catalytic species, this polymerization reaction can be turned “on” and “off” by adjusting parameters such as applied current, potential, and total charge passed. However, all of the conventional approaches in this technology are limited by the complexity of the systems, the use of specialized monomers, the requirement of restrictive equipment and/or a lack of responsive dynamic control.
As with traditional radical polymerization, the most robust and widely used form of regulation is through photo-polymerization. Unfortunately however, the most useful form of external stimuli, light irradiation, has not been successfully used in ATRP reactions (see, e.g. M. Tanabe, et al. Nat Mater 2006, 5, 467-470; Y. Kwak, et al. Macromolecules 2010, 43, 5180-5183). While some photoinitiatable ATRP reactions have been developed, in these studies, only the initiation step was photocontrolled. Consequently, all subsequent growth steps in such reactions could not be photoregulated. Moreover, attempts to develop polymerization processes that are controllable by UV irradiation have been hampered by poor control and broad molecular weight distributions (see, e.g. T. Otsu, et al. Makromol. Chem. Rapid Commun. 1982, 3, 127-132).
An ability to adapt new polymerization processes to the large number of technologies that utilize polymer structures (e.g. three dimensional microfabrication technologies such as photolithography) is highly desirable. For this reason, the development of a photo-controlled radical polymerization process represents a significant breakthrough in this technical field.